1. Field of the Invention
The present invention relates to a ceramic heater for use in, for example, a glow plug, as well as to a method for manufacturing the ceramic heater.
2. Description of the Related Art
A conventional ceramic heater for use in, for example, a ceramic glow plug includes an insulating ceramic substrate and a resistance heating member embedded in the ceramic substrate and formed of, for example, a conductive ceramic material. Because of excellent thermal shock resistance and high temperature strength, silicon nitride ceramic is a popular material for the ceramic substrate.
As in the case of manufacture of many other ceramics, manufacture of silicon nitride ceramic employs a sintering aid. A sintering aid melts into a liquid phase during firing to thereby accelerate densification of a ceramic sintered body to be obtained, and plays a role in forming a grain boundary phase for bonding silicon nitride main phase (grains) of the ceramic sintered body. Sintering aids which are commonly used in manufacturing silicon nitride ceramic include magnesium oxide (MgO) and a combination of alumina (Al2O3) and yttria (Y2O3). However, these sintering aids involve a drawback in that a vitric grain boundary phase, whose softening point is low, tends to be formed in firing, and thus high temperature strength, particularly strength at 1200xc2x0 C. or higher, of an obtained sintered body tends to be impaired. When a rare-earth metal oxide and silica are added as sintering aids, the grain boundary phase can be crystallized to thereby contribute to enhancement of high temperature strength of a sintered body. However, since a liquid phase generated during firing exhibits poor fluidity, the sintering speed lowers. As a result, nonuniform sintering tends to occur, and thus variations in room temperature strength tend to occur among sintered bodies.
In order to cope with the above problems, Japanese Patent No. 2735721 discloses a silicon nitride ceramic heater whose manufacture employs a rare-earth metal oxide and alumina as sintering aids. Conceivably, addition of Al2O3 appropriately improves the sintering speed and increases the strength of the grain boundary phase.
However, in the technique disclosed in the above publication, since Al2O3 is added uniformly to the entirety of the ceramic sintered body, the softening point of the grain boundary phase tends to lower. Thus, high temperature strength unavoidably lowers. Also, since an Al component is contained in the form of Al2O3, the grain boundary phase tends to be vitrified, causing an adverse effect on the intended improvement of high temperature strength.
The present invention provides a ceramic heater comprising a silicon nitride ceramic substrate (hereinafter referred to as either a ceramic substrate or a substrate) and a resistance heating member embedded in the silicon nitride ceramic substrate. An Al-thickened layer is formed in a surface layer portion of the silicon nitride ceramic substrate. The Al-thickened layer has an Al concentration higher than that of an internal layer portion of the silicon nitride ceramic substrate.
According to the above configuration, the Al-thickened layer is formed merely in a surface layer portion of the ceramic substrate. Thus, even when the Al component causes lowering of the softening point of the grain boundary phase, the influence is limited to the surface layer portion of the ceramic substrate. Therefore, the high temperature strength of the ceramic substrate is unlikely to be impaired. Formation of the Al-thickened layer suppresses growth of silicon nitride main phase (hereinafter, referred to as may be called merely main phase) grains in the surface layer portion of the ceramic substrate, so that abnormally grown grains which provide a starting point of fracture are hardly produced. Therefore, the Al-thickened layer prevents variations in strength, particularly room temperature strength, among ceramic substrates.
Preferably, the thickness of the Al-thickened layer is 50 xcexcm to 1000 xcexcm. When the thickness is less than 50 xcexcm, variations in room temperature strength among silicon nitride ceramic substrates may fail to be effectively suppressed. When the thickness is in excess of 1000 xcexcm, sufficient high temperature strength may fail to be imparted to the ceramic substrate. More preferably, the thickness is 50 xcexcm to 500 xcexcm. Preferably, the Al-thickened layer assumes an average Al concentration of 0.1% to 5% by weight. When the Al concentration is less than 1% by weight, growth of main phase grains in the surface layer portion of the ceramic substrate may fail to be effectively suppressed, potentially causing variations in room temperature strength among ceramic substrates. When the Al concentration is in excess of 5% by weight, the high temperature strength of the Al-thickened layer itself may be impaired, potentially failing to attain enhancement of strength of the ceramic substrate intended by means of the Al-thickened layer.
When growth of main phase grains in the Al-thickened layer is effectively suppressed, the average grain size of the silicon nitride main phase in the Al-thickened layer becomes smaller than that of the silicon nitride main phase in an internal layer portion of the ceramic substrate. Preferably, the average grain size of the silicon nitride main phase in the Al-thickened layer is 0.1 xcexcm to 1 xcexcm, and that of the silicon nitride main phase in the internal layer portion is 0.2 xcexcm to 5 xcexcm. In either case, when the average grain size is below the lower limit, preparation of material powder for attainment of the average grain size becomes very difficult. When the average grain size is in excess of the upper limit, the strength of the ceramic substrate may become insufficient In order to suppress formation of a starting point of fracture, the maximum grain size of the main phase in the Al-thickened layer is preferably not greater than 10 xcexcm. Herein, the grain size is defined as follows. Various pairs of parallel lines are drawn tangent to the contour of a crystal grain observed on a sectional microstructure of the ceramic substrate, in such a manner as not to traverse the crystal grain. The distance between the parallel lines of each pair is measured. The maximum distance is defined as the grain size of the crystal grain.
The silicon nitride ceramic substrate assumes, for example, a microstructure such that Si3N4 grains are bonded by means of a grain boundary phase (bonding phase) derived from a sintering aid component, which will be described later. Preferably, the main phase is predominantly composed of an Si3N4 phase which contains xcex2-Si3N4 in an amount of not less than 70% by volume (preferably, not less than 90% by volume). In this case, the Si3N4 phase may be such that a portion of Si or N atoms may be replaced with Al or oxygen atoms and such that metallic atoms of, for example, Li, Ca, Mg, or Y, are incorporated into the phase in the form of solid solution. Examples of such a phase construction are sialon expressed by the following formulas.
xcex2-sialon: Si6-Si6AlzOzN8-z(z=0 to 4.2) xcex1-sialon: Mx(Si,Al)12(O,N)16(x=0 to 2) 
where M represents Li, Mg, Ca, Y, or R (a rare-earth element other than La and Ce). Herein, unless otherwise specified, the term xe2x80x9cpredominantxe2x80x9d or xe2x80x9cpredominantlyxe2x80x9d used in relation to content means that the content of a substance in question is contained in an amount of not less than 50% by weight.
Preferably, a predominate portion of the Al component in the ceramic substrate is present in the form of an inorganic compound other than Al2O3. Specifically, it is preferable that the Al component be integrated into the main phase through replacing a silicon component as mentioned above and that the Al component present in the grain boundary phase assume the form of nitride or oxynitride or the form of composite nitride, composite oxide, or composite oxynitride with another sintering aid component. The Al component present in such a form suppresses vitrification of the grain boundary phase or further enhances suppression of growth of crystal grains in the main phase, thereby effectively suppressing impairment in the high temperature strength of the ceramic substrate. This effect is developed irrespective of whether or not the surface layer portion of the ceramic substrate is thickened with the Al component.
The present invention further provides a ceramic heater comprising a silicon nitride ceramic substrate containing an Al component and a resistance heating member embedded in the silicon nitride ceramic substrate. The Al component is present in the silicon nitride ceramic substrate predominantly in the form of an inorganic compound other than Al2O3.
Next, a sintering aid component is predominantly engaged in formation of the bonding phase; however, a portion of the sintering aid component may be integrated into the main phase. The bonding phase may contain unavoidable impurities; for example, silicon oxide contained in silicon nitride material powder, in addition to an intentionally added component serving as a sintering aid.
A sintering aid component usable in the present invention is not limited to a rare-earth component. For example, elements of Groups 4A, 5A, 3B, and 4B of the Periodic Table, such as Si and Al, can be used to such an extent as not to impair the effect of the present invention. The silicon nitride ceramic substrate to be obtained may contain a sintering aid component in an amount of 3% to 15% by weight on an element basis. When the content of the sintering aid component is less than 3% by weight, a dense sintered body is hardly obtained. When the content of the sintering aid component is in excess of 15% by weight, strength, toughness, or heat resistance may become insufficient. The content of the sintering aid component is preferably 6% to 10% by weight.
Rare-earth components usable in the present invention are Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In particular, Tb, Dy, Ho, Er, Tm, and Yb can be favorably used, since the elements, when added, accelerate crystallization of the grain boundary phase and improve high temperature strength. Combined addition of a rare-earth component and the Al component synergistically suppresses abnormal growth of crystal grains in the main phase and variations in room temperature strength among ceramic substrates (in particular, the synergistic suppression becomes more effective when one or more rare-earth elements among Tb, Dy, Ho, Er, Tm, and Yb are used). Conceivably, this is because presence of the above rare-earth element(s) facilitates presence of the Al component in the form of an inorganic compound other than Al2O3. The synergistic effect is maximally yielded when the Al content of the ceramic substrate is 0.1% to 5% by weight, and the rare-earth component content of the ceramic substrate is 3% to 15% by weight.
In order to accelerate crystallization of the grain boundary phase, it is preferable that a rare-earth component serving as a sintering aid component be present in the form of a composite oxide with Si; specifically, the rare-earth component be contained in the form of R2SiO5 and/or R2Si2O7, where R is the rare-earth element. Whether or not the composite oxides are contained in a crystalline form can be confirmed by various known processes of crystal analysis; for example, X-ray diffractometry for measuring diffraction patterns, or selected area diffractometry using a transmission electron microscope.
In order to accelerate crystallization of the grain boundary phase, an excess silicon oxide component which is not involved in formation of the main phase must be contained in a necessary and sufficient amount. The excess silicon oxide component accelerates, for example, formation of crystals of the aforementioned composite oxide. The content of the excess silicon oxide component can be estimated from the amount of excess oxygen, which will be described below. The amount of sintering aid components, excluding Si and Al, contained in the ceramic substrate and the total oxygen content of the ceramic substrate are obtained. The amount of oxygen required for all the sintering aid components to be present in the form of oxide is subtracted from the total oxygen content. The thus-obtained amount of oxygen is defined as the amount of excess oxygen. In order to reliably yield the aforementioned effect through addition of a rare-earth component as a sintering aid component, the amount of excess oxygen is preferably 1% to 10% by weight on an SiO2 basis. When the amount of excess oxygen is less than 1% by weight on the SiO2 basis, sinterability is impaired. When the amount of excess oxygen is in excess of 10% by weight, the softening point of the grain boundary phase drops, potentially resulting in impairment in high temperature strength.
Preferably, the Al-thickened layer assumes such a graded composition structure that the Al concentration increases toward the surface of the ceramic substrate. Through employment of the structure, a compositionally discontinuous boundary portion is unlikely to be formed between the Al-thickened layer and the internal layer portion of the ceramic substrate, thereby further enhancing the strength of the ceramic substrate.
When a gradient is present in the Al concentration of the Al-thickened layer, the thickness t of the Al-thickened layer is defined as shown in FIG. 11. Specifically, an Al concentration curve is obtained through measurement in the direction of depth x from the surface of the silicon nitride ceramic substrate. The Al concentration curve is expressed by
C=F(x) . . . xe2x80x83xe2x80x83(1) 
On the curve (1), CO is an Al concentration by weight measured on the surface of the substrate, and CB is an average Al concentration by weight of an internal portion of the substrate (CB may be substantially zero in some cases). A straight line is defined as follows.
C=CB+0.5 (COxe2x88x92CB) . . .xe2x80x83xe2x80x83(2) 
On the x-C plane, the intersection B of the curve (1) and the straight line (2) is obtained (when a plurality of intersections are formed due to influence of noise appearing on the curve (1), the one closest to the substrate surface is selected). The x coordinate of the intersection B is defined as the thickness t of the Al-thickened layer. The Al concentration curve can be obtained by a so-called line analysis. In this line analysis, an analysis line AL is first defined on the cross section of the substrate in the above-mentioned x direction, and the concentration of Al is determined along the analysis line AL by making use of an electron probe X-ray microanalyzer (EPMA). The Al concentration curve is obtained as a profile of variation in characteristic X-ray intensity of the Al component along the analysis line AL. Herein, the characteristic X-ray intensity of the Al component is considered proportional to the Al concentration.
When a gradient is present in the Al concentration in the Al-thickened layer, the ratio CB/CO (hereinafter, may be called gradient) is preferably not greater than 0.9.
The above-mentioned ceramic heater having the Al-thickened layer can be manufactured by the following method of the present invention. The method comprises the step of firing a green body or a calcined body of the silicon nitride ceramic substrate while an Al component source substance is in contact with the surface of the green body or calcined body, to thereby form in a surface layer portion of the obtained silicon nitride ceramic substrate an Al-thickened layer having an Al concentration higher than that of an internal layer portion of the silicon nitride ceramic substrate. The method has the following advantages. The Al component is diffused from the surface of the substrate to thereby easily form the Al-thickened layer. Firing and formation of the Al-thickened layer can be carried out in a single process, thereby enhancing work efficiency. The Al concentration of the Al-thickened layer can be easily adjusted. Through unidirectional diffusion of the Al component, a gradient can be easily introduced in the Al concentration of the Al-thickened layer.
The above-mentioned firing process may comprise the steps of: forming a coating layer containing the Al component source substance on the surface of a cavity of a pressing die; and hot pressing the green body or the calcined body by use of the pressing die. A parting component may be contained in the coating layer containing the Al component source substance. In this case, in a process for applying a parting material to the surface of the pressing die, the Al component source substance can be simultaneously applied to the die surface, thereby shortening the manufacturing process.
In order to contain the Al component in the ceramic substrate in the form of an inorganic compound other than Al2O3 as mentioned previously, firing can be performed at a temperature not lower than 1700xc2x0 C. in a nitrogen atmosphere having a partial pressure of oxygen (contained, for example, as an impurity) of 0.01 Pa to 100 Pa and a partial pressure of nitrogen not lower than approximately 5xc3x97104 Pa (approx. 0.5 atmosphere).
Thus, it is an object of the present invention is to provide a ceramic heater whose silicon nitride ceramic substrate substantially maintains high temperature strength without impairment resulting from addition of an Al component, and featuring low variations in room temperature strength among silicon nitride ceramic substrates.
Another object of the present invention is to provide a method for manufacturing the above-described ceramic heater.
Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings